Ceramic capacitor comprising semiconductive barium titanate body and silver alloy electrodes containing minor amounts of lead oxide and bismuth oxide

ABSTRACT

A ceramic voltage dependent resistor made up of a semiconductive titanate (e.g. barium titanate) plate having a nonohmic electrode on one surface thereof and an ohmic electrode on the other surface. The nonohmic electrode consists essentially of 93-55 percent by weight of silver component, 6.5-25 percent by weight of lead oxide and bismuth oxide and 0.5-38.5 percent by weight of at least one metal oxide selected from the group consisting of boron, copper, cadmium, zinc, nickel, cobalt, iron, titanium, niobium and tantalum. The thickness of the nonohmic electrode after firing it onto the titanate plate is 40-250 microns. The titanate plate comprises barium titanate of a means grain size of 5 to 50 microns and has an electrical resistivity less than 10 ohm-cm. Such voltage dependent resistors are suitable for use in varistors.

United States Patent I968, Oct. 8, 1968 Japan 43/43044, 43/43207,43/70654 and 43/74056 CERAMIC CAPACITOR COMPRISING SEMICONDUCTIVE BARIUM'IITANA'IE BODY AND SILVER ALLOY ELECTRODES CONTAINING MINOR AMOUNTS OFLEAD OXIDE AND BISMUTII OXIDE 12 Claims, 1 Drawing Fig.

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I-IOll 3/08 Primary Examiner-James D. Kallam Afl0rney-Wenderoth, Lindand Ponack ABSTRACT: A ceramic voltage dependent resistor made up of asemiconductive titanate (e.g. barium titanate) plate having a nonohmicelectrode on one surface thereof and an ohmic electrode on the othersurface. The nonohmic electrode consists essentially of 9355 percent byweight of silver component, 6.5-25 percent by weight of lead oxide andbismuth oxide and 0.5,38.5 percent by weight of at least one metal oxideselected from the group consisting of boron, copper, cadmium, zinc,nickel, cobalt, iron, titanium, niobium and tantalum. The thickness ofthe nonohmic electrode after firing it onto the titanate plate is 40-250microns. The titanate plate comprises barium titanate of a means grainsize of 5 to 50 microns and has an electrical resistivity less than 10ohmcm. Such voltage dependent resistors are suitable for use invaristors.

NON-OHMIC ELECTRODE CONTAINING SILVER COMPONENT, LEAD OXIDE, SILVEROXIDE AND A FURTHER METAL OXIDE.

\\\\'\SEMICONDUCTIVE TITANATE OHMIC ELECTRODE PATENTEII JUL20 IanNON-OHMIC ELECTRODE CONTAINING SILVER COMPONENT, LEAD OXIDE, SILVEROXIDE AND A FURTHER METAL OXIDE.

\"\SEMICONDUCTIVE TITANATE OHMIC ELECTRODE INVENTORS TSUNEHARU NITTAHIROMITSU "mm KANEOMI NAGASE'. SHIGERU HAYAKAWA .I Q M6M,%zdf/ ATTORNEYSThis invention relates to a voltage dependent resistor comprisingsemiconductive titanate ceramic and conducting electrodes applied tosurfaces of said ceramic and more particul larly to a voltage dependentresistor having a current-voltage characteristic which is asymmetricalwith respect to the polarity of the applied voltage, and relates also toa method of fabrication thereof. I

There are now a number of difi'erent types of voltage dependentresistors. The asymmetrical current-voltage characteristic is closelyrelated to a potential barrier formed at the point of contact betweentwo materials having different work functions.

A voltage dependent resistor, therefore, is constructed as follows: onesurface of a semiconductor is contacted by an electrode and a potentialbarrier is formed therebetween and the other surface is contacted by anohmic electrode. in the asymmetric characteristic, the forward current(I) can be expressed as a function of applied voltage (V) as follows:

1=1.e 1) where I, and B are constants (1255551 Biiih' potential barrier.3 is the reciprocal of the slope of the log l-V curve and can be writtenas follows:

The term 3 indicates the rate of variation of voltage with thelogarithmic change in current. Therefore, ,3 is a characteristicconstant of the voltage dependent resistor when the voltage dependentresistor is used as a voltage-stabilizing device, such as a conventionalSi-varistor. Another characteristic constant is a voltage for a givencurrent that'ranges in the linear portion of the log'l-V curves. Thisvoltage is tentatively termed the varistor voltage.

An asymmetrical varistor requires a small 5, various varistor voltagesover a wide range from a high voltage to a low, a reduced leakagecurrent, a high stability with respect to humidity, time, temperatureand electric load and a low cost.

Although many efforts have been directed to the concurrent improvementof all these characteristics, entirely satisfactory results have notbeen obtained with the conventional voltage dependent resistor such as aSi-varistor.

lt has been well known that barium titanate has a relatively lowelectrical resistance at room temperature (20 C. to 30 C.) when thebarium titanate has incorporated therein a small amount of rare earthoxide such as cerium oxide, samarium oxide, and/or yttrium oxide or hasbeen fired in an atmosphere containing a small amount of oxygen. Theceramics are N-type semiconductors in which current carriers areelectrons.

When silver paste is fired on the surface of the ceramic to form anelectrode, a potential barrier is formed at the interface between theelectrode and the ceramic. Therefore, a voltage dependent resistor canbe constructed when the other surface is contacted by an ohmic electrodesuch as an indium-gallium alloy. However, it is very difficult to obtainthe required asymmetric varistor properties. The properties areattributed to the rectifying action of the potential barrier layerformed at the interface between the ceramic material and the metalelectrode. The basic concept of the potential barrier layer isexemplified by US. Pat. No. 3,419,759 entitled Capacitor ComprisingFerroelectric Ceramic with Oxidic Silver Electrodes and HeterojunctionBarrier Layer between Electrodes and Ceramic," and US. Pat. No.3,419,758 entitled Ceramic Capacitor Comprising Semiconductive BariumTitanate Body and Silver Alloy Electrodes containing Minor Amount of Cu,Cd or Bi."

It is an object of the present invention to provide a novel voltagedependent resistor comprising semiconductive titanate ceramic andelectrodes and having a high nonlinearity characterized by a small valueoffi.

lt is another object of the present invention to provide a novel voltagedependent resistor characterized by various varistor voltages, reducedleakage current, high stability with respect to humidity, time,temperature and electric load and a low cost.

It is a further object of the present invention to provide a 0 methodfor making a voltage dependent resistor characterized by a small valueof [3, various varistor voltages, reduced leakage current, and highstability with respect to humidity, time, temperature and electric load.

Such voltage dependent resistors are suitable for use in varistors.

These objects are achieved by a ceramic voltage dependent resistor whichcomprises a semiconductive titanate plate having a nonohmic electrodeapplied to one surface thereof and an ohmic electrode applied to anothersurface thereof, said nonohmic electrode consists essentially of, assolid ingredients, 93-55 percent by weight of a silver component, 6.5-25percent by weight of a combined addition of lead oxide and bismuth oxideand 0.5-38.5 percent by weight of a further addition of at least onemetal oxide selected from the group consisting of boron, copper,cadmium, zinc, cobalt, iron, titanium, niobium and tantalum. These andother features of the present invention will become apparent uponconsideration of the following description taken together with theaccompanying drawing in which the FIGURE is a sectional view of apreferred embodiment of the present invention.

Proceeding with the detailed description of the present invention, theconstruction of a ceramic voltage dependent resistor contemplated by thepresent invention will be explained with reference to the FIGURE whichshows a ceramic voltage dependent resistor according to the presentinvention.

A semiconductive titanate plate 1 has a nonohmic electrode 2 applied toone surface thereof and an ohmic electrode 3 applied to another surfacethereof. Said electrode 2 and 3 are electrically connected to lead wire4 by any suitable and available method such as soldering. The nonohmicelectrode consists essentially of, as solid ingredients, 9355 percent byweight of a silver component, 6.5-25 percent by weight of a combinedaddition of lead oxide and bismuth oxide and 0.5- 38.5 percent by weightof a further addition of at least one metal oxide selected from thegroup consisting of boron, copper, cadmium, zinc, nickel, cobalt, iron,titanium, niobium and tantalum.

It is important for achievement of a voltage dependent resistorcharacterized by a high stability with time, temperature, humidity andelectric load that said nonohmic electrode applied to one surface befired at 600 C. to 920 C. in an oxidizing atmosphere containing 2 to 100percent by volume of oxygen. In connection with the leakage current andthe B-value, a preferred composition of said nonohmic electrode consistsessentially of, as solid ingredients, -60 percent by weight of a silvercomponent, 14-24 percent by weight of a combined addition of lead oxideand bismuth oxide and 6-26 percent by weight of a further addition of atleast one metal oxide selected from the group consisting of boron,copper, cadmium, zinc, nickel, cobalt, iron, titanium, niobium andtantalum.

Said combined addition preferably consists of 40-60 percent by weight oflead oxide and 60-40 percent by weight of bismuth oxide.

Said silver component can be made of pure silver at preferably of acombination of 10-80 percent by weight of silver and 20-90 percent byweight of silver oxide. It has been discovered according to the presentinvention that said silver component should be made of a finely dividedpowder having a particle size less than 1.5 microns.

A load life test of the resultant voltage dependent resistor is greatlyimproved by giving said nonohmic electrode a thickness of 40-250 micronsin accordance with the present invention.

Among various further additions, the use of boron oxide, a combinationof cadmium oxide and copper oxide or a com bination of cadmium oxide,copper oxide and niobium oxide results in a higher stability during theload life test. Said commanner. Semiconductive barium titanate isprepared by the following method: an equimolecular mixture of titaniumdiox ide and barium carbonate is wet and admixed with 1.6 wt. percent ofsilver oxide, 0.22 wt.'percent of aluminum oxide, 0.37

bination of cadmium oxide and copper oxide consists essen- 5 wt. percentof silicon oxide and 0.3 wt. percent of titanium oxtially of 20-- 80percent by weight ogcadngium oxide and 80- ide, prejssed gnsto diszs 8mm in diameter and l mm thick, and 20 percent by weight of copper oxi ean and said combinasintere at l C or 2 hours in a nitrogen atmosphere.The tion of cadmium oxide, copper oxide and niobium oxide consinteredbody is black in color and has 1.0-0.3 ohm-cm. elecsists essentially of20-50 percent by weight of cadmium oxl o trical resistivity. The grainsize is 16 to 45 microns in diameter. ide, 20--40 percent by weight ofcopper oxide and l060 These sintered discs are then provided, on onesurface, with a percent by weight of niobium oxide. nonohmic electrodeand on another surface, with an ohmic Said Semiconductive titanate platean be f rme by using a electrode as explained hereinafter. Electrodecompositions Semiconductive titanate having an electrical resistivityless containing silver, silver oxide, a combined addition of lead than10 ohm-cm, such as barium titanate or strontium oxide and bismuth oxideand afurther addition of at least one titanate. metal oxide selectedfrom the group consisting of, boron, It has been discovered accor ing tohe pre ent in en i n copper, cadmium, zinc, nickel, cobalt, iron,titanium, niobium that said semioonductive titanate plate having a meangrain and tantalum and as shown in Table I, are repared b usin size of 5to 50 microns produces a voltage dependent resistor mixed powder, theparticles of which are 0%5 to l3 n%c r5n s characterized by a highstability during a load life test. 20 in diameter. The pigmentcompositions are mixed with a con- The Semiconductive titanate l, forexample, barium titanate ventional organic binder such as polyester tofonn a pigment. can be prepared by mixing barium carbonate and titaniumThe pigment is pait iteg on gage sgrface of fthe lsgmiconductivfldioxide in an equimolecular ratio with the addition of a small tita ateceramic, ire a 0 in air or minutes an amount of titanium dioxide,pressing the mixture into the form Cooled temperature C The thickness fa disc, and sintering at 1330 to 1400" C in a idi of the electrode is 35to 260 microns. The thus-prepared discs i atmosphere T mixture can be dd i m i are subsequently provided with an ohmic electrode on anotheride, silicon dioxide and silver oxide and, when desired, can be surfacethereof- The Ohmic electrode is P p y evaporatcaicined at 1000 c to 1150 C prior {0 Simering ing aluminum metal. in order to facilitatesoldering a lead wire The electrode composition in a powder f is mixedwith thereto, the aluminum ohmic electrode is coated with copperconvemional organic binder Such as polyester to form a Pig metal byevaporation. Electrical properties are measured by a ment. Thethus-prepared pigments are painted on one surface Per Se wen'known ofthe Semiconductive titanam ceramic and fired at C to The present ceramicvoltage dependent resistor has a linear 920C in an oxidizing atmospherecontaining 2 to 100 percent oflog curves when current 15 9 than alfew byvolume of oxygen Cooling to room tempetature (20 C to milliamperes. Forexample, the varistor voltage isdeterrnined 300 C) is then carried outin said firing atmosphere. as voltage for the forward current of 1.5 mA,and is expressed According to the present invention, the firing-on atbyThe 3 V3196 ls computed 1 and Leakage current in the reverse directionis measured by apmosphere has a very pronounced effect on the varistoraction I f ld f 8v DC of the ceramic voltage dependent resistor. Forexample, a firp 2.2 a o f h h ing atmosphere having a low oxygencontent, such as a e l 'f 'l CO6 Clem o t e Yanswr voltage m t e 40forward direction is obtained by measuring V over a range nitrogenatmosphere, inhibits the varistor action. While an is f t f 55 C t C thereferred oxidizin atmos here use ma be made of an at- 0 empera-tureo 9 pp y A load life test is done in a thermostat at 60 C and more mosPherewhich 2 to 100 percem by volume of than 95 percent (relative humidity)by applying a current of available 9 a nmogen'oxygen f Good 10milliamperes for 2000 hours and the variation in the value varistoraction is obta ned when the electrode pigment is tired 4 5 of V1.5 isexpressed by percentages at a tempenfmre rangmg from 6000 C m 920Electrical properties of the present ceramic voltage depen- Theohmcfflactmfie 3 can be formefi any Sultable metal dent resistor areshown in Table II. The pigment containing Such as alummumi Zmc copperwhch Prepared by any boron oxide has ahigh varistor voltage. Thepigments containsuitable method for example, Vacuum deposition Spray ingniobium oxide or tantalum oxide tend to result in a low meta-hing lelcctmplating Aluminum metal among these varistor voltage. A combinedaddition of lead oxide and metals results the best Ohm: eleclmdebismuthoxide and a further addition of the other metal oxide The following E ofprsemly Preferred embodl results in a higher stability with respect totime, temperature, are given by of Illustration Should not be humidityand electric load than a combined addition of only strued as limitative.Percentages are by weight. lead oxide and bismuth oxide The presentceramic voltage dependent resistor has a small EXAMPLES l-27 B and asmall temperature coefficient.

- The present ceramic voltage dependent resistor is suited forSemiconductive titanate is made in a per se conventional use intransistorized bias circuits to stabilize voltage.

TABLE I Silver component in weight percent Weight percent of solidingredients of electrode pigment Sample Silver N0. Ag A320 componentPbao B1303 B203 C020 CdO ZIiO Nlo COO F9203 Tioz Nb205 T3205 100 80 10 0100 80 10 50 50 80 10 50 50 T5 10 50 50 75 10 50 50 75 10 50 50 75 10 5010 50 50 75 lo 50 50 7 l0 5 50 75 10 50 50 T5 10 50 50 T 10 50 50 T 13TiO: Nbeos lable l-C ontinued Weight percent of solid ingredients ofelectrode pigment BLO; 13:0: Cu= CdO ZnO MO 000 F820:

555251 0.0.1128 .l. .1 ill U0" 2 .46 111 .l 1

. Silver Ag Ag:0 component PbzOr Silver component in weight percentSample No.

wwwwwwmwwwwww wmwmwwwwwwwww Electrical properties Varied value of .5m... Reverse after applying direction Temperature a current ofcoefficient rna. for 2,000 Leakage of .5 mahrs. at 60 C. current at (55C. to and 95% RH 8V. (D 0.

5 +85 C.) 1.5mm. (p n la- TABLE 11 Forward direction i-5 rna- "is ma- 1These samples are illustrations to indicate the poorer properties ofceramic voltage dependent varistors which are outside the scope of thepresent inventionv Thickness of non-ohmic electrode (micron) Sample No.

627875102371mmm073 539 75 4?o 3 0 0 0 1 LLLLO ILLIOOLZLZLZ II:

7 2 no. 204 2 n zfl1 m%wm wmwmwmm mm112%HNN3W r 000 00000000000 000000 13]. .LLLL1 I t t .2 11.131111111001111! percent by 20-40 percent by 60percent by weight of niobig to claim ccordingto can to is comprised of 0microns and an g to claim 1y of 40 40 percent by g to titanate plateconsists 8. A ceramic volt age dependent resistor a 3, wherein saidsemiconductive titanate pla titanate having a mean grain size of 5 to 5electrical resistivity less than 10 ohm-cm.

5. A ceramic voltage dependent resistor according to claim wherein saidfurther addition consists essentially of 20-80 6. A ceramic voltagedependent resistor according to claim 2, wherein said further additionconsists essentially of 20-50 60 percent by weight of cadmium oxide 7. Aceramic voltage dependent resistor accordin 2, wherein said ohmicelectrode comprises aluminu 9. A ceramic voltage dependent resistoraccordin 70 2, wherein said combined addition consists essential 10. Aceramic voltage dependent resistor accordin claim 9, wherein saidscmiconductive 4. A ceramic voltage dependent resistor according toclaim 7 essentially of barium titanate.

2, wherein said further addition consists essentially of boron oxide.

percent by weight of cadmium oxide and 80 93-55 percent by weight ofsilver weight of copper oxide. component, 6.5-25 percent by weight of acombined addi- 38.5 by weight of a further addition of at least onemetal oxide selected from copper, cadmium, zinc, nickel, weight ofcopper oxide and 10- niobium and tantalum. um oxide. 2. A ceramicvoltage dependent resistor according to claim percent by weight of leadoxide and 60- weight of bismuth oxide. -90 percent by weight Thesesamples are illustrations to indicate the poorer properties of ceramicvoltage dependent varistors which are outside the scope of the presentinvention.

What we claim is:

l. A ceramic voltage dependent resistor comprising a semiconductivetitanate plate having a nonohmic electrode on f, as solid ingredientsiron, titanium,

wherein said nonohmic electrode consists essentially of, as solidingredients, 80-60 percent weight of silver component, 14-24 percent byweight of a combined addition of lead oxide and bismuth oxide and 6-26percent by weight of a 3. A ceramic voltage dependent resistor accordingto claim 2, whereinsaid silver component consists essentially of l0- onesurface thereof and an ohmic electrode on the other surface thereof,said nonohmic electrode consisting essentially 0 tion of lead oxide andbismuth oxide and 0.5-

the group consisting of boron cobalt further addition of at least onemetal oxide from the group consisting of boron, copper, cadmium, zinc,nickel, cobalt, iron, titanium, niobium and tantalum.

percent by weight of silver and 20 of silver oxide.

addition of at least one metal oxide selected from the group consistingof boron, copper, cadmium, zinc, nickel, cobalt, iron, titanium, niobiumand tantalum; firing said electrode pigment at 600 C to 920 C in anoxidizing atmosphere having an oxygen content ranging from 2 to percentby volume; cooling said fired body having said electrode pigment thereonto room temperature in said oxidizing atmosphere; and applying toanother surface of said ceramic body an aluminum electrode.

2. A ceramic voltage dependent resistor according to claim 1, whereinsaid nonohmic electrode consists essentially of, as solid ingredients,80-60 percent weight of silver component, 14-24 percent by weight of acombined addition of lead oxide and bismuth oxide and 6-26 percent byweight of a further addition of at least one metal oxide from the groupconsisting of boron, copper, cadmium, zinc, nickel, cobalt, iron,titanium, niobium and tantalum.
 3. A ceramic voltage dependent resistoraccording to claim 2, wherein said silver component consists essentiallyof 10-80 percent by weight of silver and 20-90 percent by weight ofsilver oxide.
 4. A ceramic voltage dependent resistor according to claim2, wherein said further addition consists essentially of boron oxide. 5.A ceramic voltage dependent resistor according to claim 2, wherein saidfurther addition consists essentially of 20-80 percent by weight ofcadmium oxide and 80-20 percent by weight of copper oxide.
 6. A ceramicvoltage dependent resistor according to claim 2, wherein said furtheraddition consists essentially of 20-50 percent by weight of cadmiumoxide, 20-40 percent by weight of copper oxide and 10-60 percent byweight of niobium oxide.
 7. A ceramic voltage dependent resistoraccording to claim 2, wherein said ohmic electrode comprises aluminum.8. A ceramic voltage dependent resistor according to claim 3, whereinsaid semiconductive titanate plate is comprised of titanate having amean grain size of 5 to 50 microns and an electrical resistivity lessthan 10 ohm-cm.
 9. A ceramic voltage dependent resistor according toclaim 2, wherein said combined addition consists essentially of 40 -60percent by weight of lead oxide and 60-40 percent by weight of bismuthoxide.
 10. A ceramic voltage dependent resistor according to claim 9,wherein said semiconductive titanate plate consists essentially ofbarium titanate.
 11. A ceramic voltage dependent resistor according toclaim 2, wherein the said nonohmic electrode has a thickness of from 40to 250 microns.
 12. A method of making a ceramic voltage dependentresistor comprising providing a semiconductive titanate ceramic body;applying to one surface of said ceramic body, an electrode pigmentconsisting essentially of, as solid ingredients, 93-55 percent by weightof silver component, 6.5- 25 percent by weight oF a combined addition oflead oxide and bismuth oxide and 0.5-38.5 percent by weight of a furtheraddition of at least one metal oxide selected from the group consistingof boron, copper, cadmium, zinc, nickel, cobalt, iron, titanium, niobiumand tantalum; firing said electrode pigment at 600* C to 920* C in anoxidizing atmosphere having an oxygen content ranging from 2 to 100percent by volume; cooling said fired body having said electrode pigmentthereon to room temperature in said oxidizing atmosphere; and applyingto another surface of said ceramic body an aluminum electrode.